The present invention relates to an oscillation circuit using a field effect transistor (FET), and more particularly to an oscillation circuit suited for use in a local oscillation circuit of an SHF band receiver.
A circuit as shown in FIG. 1 has been used as a prior art oscillation circuit including an FET in a distributed constant circuit using microstrip lines. In FIG. 1, numeral 1 denotes an FET having a shape suited for use in a microstrip circuit, 2 denotes a gate terminal, 5 denotes a source terminal and 7 denotes a drain terminal. The gate terminal 2 has a length which is approximately equal to a quarter (1/4) wavelength of a desired oscillation frequency, and it is connected to a resonator 3 which comprises an open-ended microstrip line. A bias voltage is applied to the gate terminal 2 through a choke coil 4 from a biasing circuit (not shown). The source terminal 5 is connected to a microstrip line 6 having its terminal end connected to ground and acting as a feedback circuit. The terminal end of the line 6 is connected to a conductor on the backside of a substrate constituting a microstrip at an end thereof or grounded by being connected to a conductive case, as is usually done in the microstrip circuit, but in the illustrated drawing it is designated by a grounding symbol used in a conventional circuit diagram. Throughout the drawings of the present invention, the grounding portions are similarly designated. The drain terminal 7 is connected to a microstrip line 8 which conducts an oscillation output signal. The output microstrip line 8 is split at the intermediate point thereof and the respective halves are interconnected by a capacitor 10 to isolate itself from the succeeding stage D.C.-wise. A bias voltage is applied to the drain terminal 7 through a choke coil 9 from a biasing circuit (not shown).
In the circuit thus constructed, feedback is applied from the drain to the gate by the feedback circuit 6 connected to the source terminal 5 so that the circuit oscillates at a resonance frequency of the resonator 3 connected to the gate terminal 2. In the oscillation circuit, a negative bias voltage is normally applied to the gate terminal 2.
In such an oscillation circuit, the FET 1 may be connected in a grounded drain configuration as shown in FIG. 2. In the case of the grounded drain configuration, no external feedback path is necessary because the amount of feedback between the gate and the source within the FET is large. Accordingly, as shown in FIG. 2, the drain terminal 7 may be grounded and an output line 11 may be connected to the source terminal 5. The capacitor 10 is inserted at the intermediate point of the output line 11 to block D.C. In this circuit, negative voltages are required for the bias voltage applied to the gate terminal through the choke coil 4 and the bias voltage applied to the source terminal 5 through a choke coil 12.
In the oscillation circuits shown in FIGS. 1 and 2, an oscillation condition is met by a relation between impedances of the FET 1 and the resonator 3 viewed from the gate terminal 2. As is well known, an impedance of an open-ended line changes with a relation between a length of the line and a wavelength and with a frequency. Further, there exist many frequencies, not one, which cause the same impedance to appear. As a result, if the impedance of the FET changes by the change of ambient temperature, the oscillation frequency changes greatly or jumps to an undesirable frequency.